Variation in Assemblages of Freshwater Mussels Downstream of Dams and Dam Removals in the Lake Michigan Basin, Michigan, USA

Impacts of barriers, including dam removals, on aquatic diversity are poorly understood. We assessed freshwater mussel assemblages and microhabitat downstream of uncontrolled and controlled low-head dams and low-head dam removals in Michigan, USA. The objectives of this study were to quantify whether downstream mussel assemblages and microhabitat parameters differ by anthropogenic barrier and along a downstream gradient, and to determine parameters that were predictors of mussel diversity and density. Sampling consisted of standardized timed-searches and quadrat excavations. Results suggest that areas downstream of dams had higher mussel diversity than dam removals, and mussel assemblages differed along a downstream gradient for uncontrolled and controlled dams. Indicator Species Analyses determined mussel species representative of downstream river reaches from uncontrolled low-head dams and removals. Predictor variables for mussel assemblages included substrate classes and total suspended solids. Controlled dams contained the least fine substrates (%) and highest coarse substrates (%) in downstream reaches. This study suggests that rivers with uncontrolled low-head dams and removals provide downstream habitat that support viable mussel assemblages. Results from this study also suggest that evidence of mussel assemblage recovery following dam removal may take many years. Quantification of barrier-related impacts, as shown in this study, are imperative to guide conservation of aquatic fauna.

Watershed Suspended Sediment Supply and Potential Impacts of Dam Removals for an Estuary

Observations and modeling are used to assess potential impacts of sediment releases due to dam removals on the Hudson River estuary. Watershed sediment loads are calculated based on sediment-discharge rating curves for gauges covering 80% of the watershed area. The annual average sediment load to the estuary is 1.2 Mt, of which about 0.6 Mt comes from side tributaries. Sediment yield varies inversely with watershed area, with regional trends that are consistent with substrate erodibility. Geophysical and sedimentological surveys in seven subwatersheds of the Lower Hudson were conducted to estimate the mass and composition of sediment trapped behind dams. Impoundments were classified as (1) active sediment traps, (2) run-of-river sites not actively trapping sediment, and (3) dammed natural lakes and spring-fed ponds. Based on this categorization and impoundment attributes from a dam inventory database, the total mass of impounded sediment in the Lower Hudson watershed is estimated as 4.9 ± 1.9 Mt. This represents about 4 years of annual watershed supply, which is small compared with some individual dam removals and is not practically available given current dam removal rates. More than half of dams impound drainage areas less than 1 km2, and play little role in downstream sediment supply. In modeling of a simulated dam removal, suspended sediment in the estuary increases modestly near the source during discharge events, but otherwise effects on suspended sediment are minimal. Fine-grained sediment deposits broadly along the estuary and coarser sediment deposits near the source, with transport distance inversely related to settling velocity.

Rethinking Hydropower: The Economics and Politics of Privately Owned Hydropower in the United States

2019 marked the 20th anniversary of the removal of the Edwards Dam in Augusta, Maine (USA). Edwards Dam was the first federally licensed hydropower dam to be denied relicensing, and the dam was removed for the purpose of restoring the 10 anadromous fish species that use the Kennebec River. Since that time, numerous other small dams have been removed in the United States. The relicensing process considers benefit-cost analysis, yet remains fundamentally flawed in the consideration of the benefits of dam removals and fish passage. Successful dam removals rely (mostly) on local efforts and outside analysis.

Managing Centrarchid Fisheries in Rivers and Streams

<em>Abstract.</em>—Fish assemblages in Atlantic coastal rivers have undergone extensive ecological change in the last two and a half centuries due to human influence, including extirpation of many migratory fish species, such as river herring <em> Alosa </em>spp. and introduction of nonnative piscivores, notably Smallmouth Bass <em> Micropterus dolomieu</em>. Recently, dam removals and fish passage improvements in the Penobscot River, Maine, have allowed river herring to return to reaches of the river that have been inaccessible since the late 19th century. Alosine populations have increased and this trend is anticipated to continue. This may increase forage in the system which could potentially increase growth for Smallmouth Bass, the dominant piscivore. We examined the diet and growth of Smallmouth Bass collected from areas of the Penobscot River watershed with and without access to river herring as prey. We collected 765 Smallmouth Bass throughout 2015, examined the stomach contents of 573 individuals, and found notable differences in diet among three river reaches with common seasonal trends. Juvenile river herring composed an average of 19% (SE = ±6%) of stomach contents by mass from Smallmouth Bass collected in the freshwater tidal area but were rarely observed in the diets upstream. We used estimates from von Bertalanffy growth models to examine differences in growth among reaches and found that asymptotic length was the longest (425 mm TL) in the Tidal reach where access to river herring was unrestricted. We then used these data to predict changes to growth associated with increased access to juvenile river herring prey with bioenergetics models. Results indicated that substituting juvenile river herring for less energy-dense prey (e.g., invertebrates) may lead to increases in seasonal growth throughout the watershed as river herring populations continue to rebound in response to dam removal. Our results provide insight into the diet and growth of Smallmouth Bass in a large New England river, and provide a foundation for future work investigating unfolding changes to these characteristics following recent dam removals.